As illustrated in FIG. 1, prior art pMTJ (perpendicular Magnetic Tunnel Junction) 10 includes a free layer 11, junction layer 12 and reference layer 13. The junction layer can also be referred to as a tunneling barrier layer. The magnetization direction of the free layer 11 is switchable as illustrated by the arrow between the up and down directions with respect to the plane of the thin films that are deposited and patterned on wafers to form the structures. Reference layer 13 has a fixed magnetization direction, which in this example is illustrated by the upward pointing arrow. On the left hand side of FIG. 1 the magnetization direction of the free layer 11 is antiparallel to that of the reference layer 13, which corresponds to the high electrical resistance state. In the prior art the free layer is switching from antiparallel to parallel by driving the flow of electrons from the reference layer 13 to the free layer 11. FIG. 2 is an illustration of the switching of the free layer in a prior art pMTJ memory element from parallel to antiparallel by driving the flow of electrons from the free layer to the reference layer. The use of opposite electron flows will be referred to as bipolar current switching. In prior art STT-MRAM the magnetic switching in both directions occurs by spin transfer torque (STT). The antiparallel to parallel switching is achieved by electrons carrying spin information from the reference layer to the free layer. The parallel to antiparallel switching is achieved by STT from electrons being reflected back from reference layer.
Traditional STT-MRAM architecture uses cells that include one driver transistor for each MTJ (1-transistor+1-MTJ) as illustrated in FIG. 7. STT-MRAM area data density is limited due to the limitation on transistor current that is required to switch the magnetization of the free layer of the MTJ device, whereas a larger size transistor is needed to achieve the current level required to switch the MTJ. The transistor, therefore, occupies a larger in-plane area of the substrate. The in-plane direction of the substrate in FIG. 7 is horizontal. In contrast a diode has a much higher current limit for a comparable cross section area; therefore, a 1-Diode+1-MTJ architecture has advantages for minimal cell size and higher current during switching. However, the diode architecture allows current flow in only one direction and inherently requires unipolar current switching of the bipolar magnetizations of the MTJ. In prior art designs that use (1-Diode+1-MTJ) architecture and electric field effect to switch free layer magnetizations, due to the intrinsic physics limitations, the MTJ is required to have low coercivity field of the free layer or tilted free layer magnetization to achieve bipolar magnetization switching with a unipolar current. Such limitation leads to this architecture being impractical and unreliable in real world application.